(a) Field of the Invention
The present invention relates to a process for casting steel from a first container into a second container in which the liquid metal is protected against oxidation and/or nitridation. More particularly, the present invention is directed to a process for the continuous casting of steel which includes the following consecutive steps:
the liquid steel is cast from a converter or an electrical furnace into a ladle, PA1 the liquid steel is cast from the ladle into a tundish, PA1 the liquid steel is cast from the tundish into at least one continuous casting ingot mold.
(b) Description of Prior Art
On the start of a casting of liquid steel, for example from a ladle into a tundish or during the casting from a first ladle into this tundish, in the case of a process which is carried out sequentially, the liquid metal is in contact with the atmosphere.
The height of the drop of the liquid metal in the tundish and the disturbances taking place result in rather important reactions of nitridation and/or oxidation, which generally last until the complete immersion of the gas-nozzle in the liquid metal which is cast into the tundish, this gas-nozzle being placed at the lower end of the ladle and surrounding the casting jet. After the immersion of the lower portion of the gas-nozzle, the problems resulting from nitridation and/or oxidation are less important because in general, there are used covering powders which are spread on the surface of the liquid metal present in the tundish, or any other known analogous means.
In a general manner, during a ladle-tundish casting, the nitridation and/or oxidation phenomenon mentioned above lasts from about 45 seconds to 4 minutes depending on the size and shape of the tundish. The cast metal which is present in the tundish before the immersion of the gas-nozzle is thus more or less highly oxidized and/or nitrided and the billets or ingots of steel formed from this metal do not possess the desired metallurgical properties.
Among the known processes intended to overcome these disadvantages there is a process known under the commercial name "SPAL", which has been designed by the applicants' assignee and which uses cryogenic liquids such as liquid argon or nitrogen, which very efficiently protect the impact zone of the jet of metal by rendering the bottom of the container inert before the start of the casting and by thereafter covering the surface of the liquid metal to be protected.
However, when it is also intended to produce steels with a low percentage of nitrogen, i.e. when it is intended to prevent a nitridation of steel, it is not possible to use liquid nitrogen to protect the metal melt. In this case, the only process which is actually available at present includes utilizing liquid argon which is spread on the surface of the liquid metal. However, argon is a gas which is relatively expensive and there is presently a search for a more economical solution which would enable metallurgical results which are substantially identical to those obtained when liquid argon is used.